This disclosure is another chapter in the never ending quest to find more desirable sources of energy. At the present time, the demand for energy is satisfied primarily by the consumption of fossil fuels and nuclear energy. The consumption of energy is accompanied by the creation of pollutants that are beginning to pose a serious threat to the environment. Forests are threatened by acid rain. The heat generated by the consumption of such energy sources has caused global warming, the long term effects of which are as yet unknown. For these and many more reasons, the search is on for efficient and inexpensive sources of energy without pollution.
One such source that meets this goal is the generation of electricity from sunlight. The primary component used in this process are solar cells or, more specifically, photo-voltaic devices. Photo-voltaic devices essentially create electrical current when exposed to sunlight. However, the photo-voltaic technology is not without its problems as well. Chief of these is the high cost of manufacturing photo-voltaic cells. A second problem is that such devices tend to be inefficient, the electronic energy created being only a fraction of the sunlight energy received. Consequently, less expensive methods to make more efficient photo-voltaic devices are desirable.
As shown in FIG. 1A, photo-voltaic cell is made of a silicon wafer 5 that has been doped with a base dopant material. The sides of the wafer 5 are then diffused with dopant material forming two diffused regions 10. In a sense, the wafer will have internal layers of diffused material and non-diffused material. The wafer is then covered with a layer of oxide 15 on both sides which enhances the photo-voltaic effect by passivating the surfaces. The final steps include the addition of a non-reflective coating 25 to ensure the absorption of sunlight and the introduction of contacts layers 30 and 35 which are connected to the diffused layers 10 which are used to connect the solar cell to an electrical load.
There are various conventional methods used in creating these photocells that yield certain photo-cell efficiencies. Among the processes that yield the greatest efficiencies is that which is described next.
The first step in this process is to clean the silicon wafers from which the solar cells are produced. This cleaning process is quite extensive as will be discussed. Next, a layer of masking oxide is grown on both sides of the silicon wafers. This step is performed in a furnace at a predetermined temperature. Oxygen is introduced into the heated furnace and over time a layer of oxide grows on the sides of the cell wafers. This is a common procedure that is within the understanding of those skilled in the art.
After the masking layer is grown, it is removed from the rear side of the wafer. This is done by an etching process known to those skilled in the art in which the oxide layer is removed. After the layer of oxide on the rear side has been removed, the cell wafer is cleaned in an involved process known to those skilled in the art using powerful chemicals to remove contaminants from the surfaces of the wafer. This procedure can be quite time consuming adding significant cost to the procedure.
After the layer of oxide is removed from the rear side, the next operation is also the second furnace cycle in which the wafers are placed into a furnace with the etched rear side exposed to receive the diffusion of dopant. The oxide coating remaining on the front side prevents the dopant from diffusing into the front side of the cell wafer. The furnace is then heated to a predetermined temperature and the dopant is introduced into the environment. At elevated temperatures, the dopant will diffuse into the exposed rear side of the wafer. The depth of the diffusion into the wafer is known as the "junction depth" and is a function of the temperature and time of the heating cycle. All of the above operations require absolute cleanliness to prevent contaminants from being diffused into the cell wafer along with the dopant.
While the rear side is diffused with dopant in the second thermal step above, a thick glass-like layer is formed over both sides of the wafer. This layer is a byproduct created by excess dopant in the glass. In this step, the wafer is cooled down and the glass layer is removed in another etching process to prevent further diffused during subsequent thermal cycles. The layer of oxide on the opposite side of the wafer is etched off as well. As before, this etching step provides an opportunity for impurities to be introduced into the overall process and also incurs additional time and expense due to materials handling and consumption of cleaning chemicals. Also lost is any potential for surface passivation when etching the oxide as known to those skilled in the art.
The next operation is to clean the wafers after the etching. As before, this cleaning is quite extensive requiring several chemicals including acids. Consequently, the cleaning step poses a threat to the environment due to the necessary disposal of these cleaning chemicals. Also, the cleaning process is quite time consuming incurring additional time and expense in the overall process.
The next step involves the third furnace cycle in which a layer of masking oxide is once again grown on both sides of the cell wafer. This step is much like the first step with the additional time and expense incurred. After the oxide layer is grown, this time the oxide is removed from the front side by etching and the oxide on the back acts as a diffusion mask. As before, the wafer will undergo an extensive cleaning process after the etching is complete.
The operation that follows is the fourth furnace cycle in which the front side of the cell wafer is diffused with dopant. Once again, the wafers are placed in a furnace with the front side exposed and the temperature is raised to the predetermined level. The dopant is then introduced into the environment and is diffused into the front side of the wafer. As with the dopant diffused into the rear side, the junction depth of the front side is a function of the time and temperature of the heating cycle. Also, as was the case with the rear side diffused, a thick glass-like substance is formed on both sides of the wafer as a byproduct of the diffusion process.
Not surprisingly, the next operation will involve the etching of this second glass-like layer and the oxide covering the front and rear sides of the cell wafer since the diffusion glass contains a high concentration of excess dopant. As with all previous etching cycles, contaminants will have an opportunity to be introduced into the process. Also, additional time and expense are incurred.
This is followed by another extensive chemical cleaning cycle. The time and expense necessary to clean the wafer is incurred once again. Also, contaminants are given another opportunity to enter the process.
The next step involves the thermal oxidation of the solar cell wafers. In this step the doped wafers are once again placed in a furnace and heated to a predetermined temperature. Oxygen is then injected into the furnace which reacts with the silicon to form silicon dioxide, SiO.sub.2, on both surfaces of the cell wafer. The layer of oxide that results enhances the photo-voltaic process by reducing the loss of sunlight generated carriers or electron-hole pairs into the silicon surface. Also, during the oxidation process, any contaminants that have found their way into the process may be diffused into the cell wafers.
After the thermal oxidation has taken place, ohmic contacts are placed on the solar cell. The surface on which sunlight is to be absorbed is fitted with a grid of contacts which are in direct contact with the front diffused layer of the cell wafer. This grid ensures uniform collection of electrical charge while allowing sunlight to shine into the cell itself. The reverse side is fitted with a solid conductor over the back oxide which makes contact with the back diffusion through "points" patterned in the oxide. For lower efficiency cell designs, the back oxide could be removed completely and metal may be deposited on the entire back surface.
The final operation involves the creation of an anti-reflection coating on the front surface through which sunlight is gathered. This coating ensures maximum energy output by preventing sunlight from being reflected rather than absorbed by the solar cell.
While the above process can create relatively efficient photocells, it is not without difficulties. First is the problem of preventing contaminants from entering the process, especially during the five furnace cycles. Contaminants present in the cell wafer can significantly degrade the performance of photocells. They may find their way onto the cell wafer at any time during the process. Once the cell wafer is placed in a furnace, any contaminants that are present either on the cell wafer or in the furnace can be diffused into the cell wafer at greater temperatures. Also, contaminants that are already inside the wafer move around and may decorate other defects such as dislocations as known to those skilled in the art. In order to prevent the degradation in photo cell performance due to contaminants, all of the process steps are performed in a very clean environment such as a clean room. This type of environment is extremely expensive to maintain thereby making it impractical to mass produce high efficiency cells at a relatively low cost with this process.
Another problem that accompanies this method of photocell manufacture is the creation of structural defects due to the multiple furnace cycles. When the cell wafers are heated, a certain amount of stress relief will occur. The relief of stresses in the cell material may cause dislocations and other defects in the wafer material that effect their efficiency. Consequently, it is desirable to have as few furnace cycles as is possible.
The foregoing process is not the only method by which solar cells are created. There are other methods which are less time consuming yielding lesser efficiencies that are known to those skilled in the art.
Consequently, the foregoing illustrates a need for a low cost method of manufacturing high efficiency solar cells featuring as few steps as possible to minimize handling and opportunities for contaminants to be introduced into the process. In particular, such a method should require as few furnace cycles as possible to prevent contamination and degradation of the cell wafers.